The major source of electricity is achieved by coal fired power plants. There are vast supplies of coal in the United States and throughout the world, and the power potentially available from coal combustion is enormous.
However, although coal is abundant and relatively inexpensive as an energy source, much of it is becoming increasingly undesirable because of the airborne emissions its produces. In particular, sulfur oxides (SO.sub.x) and nitric oxides (NO.sub.x), when emitted into the atmosphere, combine with moisture to create sulfuric and nitric acids, the chief components of acid rain. Still further, coal combustion also produces pollutants in the form of solid particulates.
Waste incineration likewise creates undesirable levels of SO.sub.x and NO.sub.x, and of solid particulates.
Over the next decade, both generating plants and waste incinerators will be required by law to make major reductions in their levels of emissions. With the technology currently available, coal burning plants have two primary methods of reducing SO.sub.x, emissions:
The first method is to burn coal with lower sulfur content. However, low sulfur coal is appreciably more expensive than medium and high sulfur coals, and it will become more so as demand increases. Further, decreasing the sulfur content has no effect on NO.sub.x, emissions.
The second method for reducing SO.sub.x emissions is scrubbing, such as embodied in the process of treating the combustion flue gases with limestone and converting the SO.sub.x to calcium sulfate.
Scrubbing can be effective in reducing SO.sub.x emissions. However, scrubbing equipment is expensive to install, operate and maintain. Moreover, the power required to run the scrubbing equipment imposes substantial parasitic energy losses. When limestone is used in the combustion chamber, as it is in fluidized bed combustors (the newest and cleanest type of system), it reduces the efficiency of the combustion process itself. Finally, limestone scrubbing does not lower NO.sub.x emissions.
NO.sub.x is currently reduced in two ways:
The first way is to modify the combustion process mechanically by installing special low NO.sub.x, burners. However, the capital costs are high, installation can require long shutdowns, and the degree of NO.sub.x reduction is limited.
The second way is to treat the flue gases with either ammonia or urea sprays. This, too, is effective, but the systems are expensive to operate, and ammonia can sometimes itself become a secondary pollutant.
Particulate matter is currently removed from flue gases by means of cyclone extraction, filtering, electrostatic precipitation, or a combination thereof. In such cases, the equipment is expensive to build, operate and maintain, and it imposes parasitic energy losses.
In effect, reducing the three forms of undesirable emissions, SO.sub.x, NO.sub.x and particulates requires three separate processes, all of them expensive.
Waste incineration is currently facing the same type of regulatory problems as coal. Municipalities and states are insisting that SO.sub.x, NO.sub.x and particulate emissions be reduced substantially below present levels and since the low sulfur fuels available to coal burning plants have no counterpart in waste incineration, scrubbing and NO.sub.x treatments are at present the only practical methods.
Discarded automobile tires are a particular problem. They cannot be dealt with satisfactorily in land fills, and although their high carbon content makes them potentially an excellent energy source, their high sulfur content makes ordinary burning of them effectively disadvantageous. Furthermore, reducing the level of pollution can consume a large portion of the energy that is generated.
There exists, therefore, a need for a process for burning combustible, sulfur containing carbonaceous materials or organic materials, such as coal, rubber and waste materials, such as municipal waste materials which does not exhibit the above mentioned disadvantages.
The present invention fulfills this need.